Many thin image display devices as typified by displays, i.e., flat panel displays, have been released on the market, because they are thinner than cathode-ray tube displays and they do not occupy much space in depth. Their market price has decreased year by year with advances in production techniques, resulting in a further increase in demand and a yearly increase in production. Especially, color LCD TVs have almost become the mainstream of TVs. Also in recent years, organic light-emitting display devices such as organic EL displays, which emit light by themselves and thereby have high visibility, have received attention as the next generation of image display devices. In relation to the performance of these image display devices, there is a strong demand for a further increase in image quality, such as an increase in contrast and color reproducibility, and a decrease in power consumption.
A color filter is used in these liquid crystal display devices and organic light-emitting display devices. For example, in the case of color LCDs, the amount of light is controlled by using a back light as the light source and electrically driving the liquid crystal. Colors are represented by the light that passed through the color filter. Accordingly, the color filter is indispensable for color representation in LCD TVs and plays a large role in determining display performance. In organic light-emitting display devices, a color image is formed in the same manner as liquid crystal display devices, when the color filter is used in combination with an organic, white light-emitting element.
A recent trend is that there is a demand for power-saving image display devices. To increase backlight use efficiency, there is a very high demand for high-luminance color filters. This is a major issue especially for mobile displays such as mobile phones, smart phones and tablet PCs.
Even though technological advances have increased battery capacity, there is still a limit on battery capacity of mobile devices. Meanwhile, there is a trend that power consumption has grown with the increase in screen size. Image display devices containing a color filter determine the design and performance of mobile terminal devices, because they are directly linked to the usable time and charging frequency of mobile terminal devices.
In general, a color filter has a transparent substrate, color layers made of color patterns of the three primary colors (red, green and blue), and a light shielding part formed on the transparent substrate so as to define each color pattern.
To form such color layers, a pigment dispersion method in which pigments with excellent heat resistance and light resistance are used as color materials, has been widely used. However, it is difficult for color filters produced by use of pigments to satisfy the latest demand for higher luminance.
As a means to achieve higher luminance, color resin compositions for color filters, which contain dyes, have been studied. Compared to pigments, dyes generally have a higher transmittance and are able to produce a high-luminance color filter. However, dyes are problematic in that they are inferior in heat resistance to pigments and the chromaticity is likely to change when they are heated at high temperature in color filter production process, for example. Also, regarding color resin compositions for color filters, there is also a demand for improvement in alkaline resistance that a coating film is not peeled off in an alkaline aqueous solution in production process.
As a means to improve various kinds of resistance properties of dyes, a method for producing a salt-forming dye is known.
For example, in Patent Literature 1, a colored photosensitive composition containing a dyed lake pigment is disclosed, and examples of the dyed lake pigment include known pigments identified by color index numbers. Also, in Patent Literature 2, a blue colored composition for a color filter, the composition including a coloring agent consisting of a copper phthalocyanine blue pigment and a metal lake pigment of a xanthene-based dye, is disclosed as a blue colored composition for a color filter, which is able to form a color filter having high luminosity and a wide color reproducing region. However, in color layers formed by using the colored compositions described in these Patent Literatures, heat resistance of the lake pigment is not sufficient.
In Patent Literature 3, as a colored composition capable of exhibiting superior chromaticity characteristics of dyes and lake pigments, even when being subjected to a high-temperature heating process, a colored composition characterized by including: a coloring agent including at least one kind selected from the group consisting of dyes and lake pigments; a polymer including a repeating unit derived from at least one kind selected from the group consisting of an ethylenically unsaturated monomer having a sulfonic group, an ethylenically unsaturated monomer having a phosphonooxy group, and salts thereof; and a crosslinking agent, is disclosed. However, in the technique of Patent Literature 3, as described in Comparative Example later, there is a problem in that dispersibility of the lake pigment is poor or heat resistance of the lake pigment is not sufficient, and alkaline resistance is also poor.